Plastic injection molding is a commonly practiced general technique for mass production of automotive lens assemblies. Processes and machinery may be adapted for production of multi-colored lens assemblies such as may be found on rear taillamp assemblies which incorporate rear marker, braking, turn signal, hazard and reverse indicators. Such multi-colored lens assemblies are generally well accepted for their final assembly advantages, styling flexibility, and aesthetically pleasing appearance.
Multi-color lens assemblies generally require sophisticated injection molding apparatus comprising multiple injection stations. That is to say, multi-colored lens assemblies are generally injection molded at sequentially progressive injection stations wherein each such station is dedicated to molding certain colors and/or features of the lens assembly.
Generally, multiple injection station molding apparatus employ automated transfer of parts from one injection station to the next sequentially progressive injection station. Certain mold injection arrangements are generally more desirable than others, particularly when transferring molded parts from one station to another in a multi-station injection molding apparatus. For example, central valve gating parts at a major surface is generally preferred to edge gating parts since additional trimming operations and cavity fill limitations are associated with edge gating operations. Additionally, the nozzle drops in valve gate operation generally provide for simpler material distribution to the cavities to be filled. However, lens assemblies are generally shaped with a smooth convex outer or viewing side exposed to the environment and a concave inner or illumination source side including structural features. Where valve gating is practiced on lens assemblies, the part geometries have necessitated valve gating on the viewing side from nozzle drops in non-indexing cover halves with material draws into the opposing indexing ejector half. The material draws advantageously provide retention of the molded part to the indexing ejector half allowing the indexing ejector half to move to the next injection station with the part intact. Valve gating lens assemblies on the viewing side, however, tends to leave injection point witness marks which are aesthetically unacceptable particularly when located in a central location of a transparent portion of the lens assembly. Additionally, particularly with respect to the preferred plastic material, polycarbonate, surface stress localized about the injection site may preclude ultraviolet protective coating application due to extremely noticeable surface blemishes which appear in the areas of the surface stress when coated. This is a disadvantage since unacceptable discoloration and deterioration of polycarbonate is known to occur when exposed to ultraviolet light.